The present invention relates to a method of mass spectrometry and a mass spectrometer. The preferred embodiment relates an improved method of calibrating ion mobility drift times which enables detailed studies of Electron Transfer Dissociation (‘ETD’) fragment ion structures using ion mobility mass spectrometry to be performed.
Ion mobility is a powerful technique for obtaining structural information for ions ranging from peptide fragments to large protein complexes. Certain secondary structures such as helices are known to have larger collision cross sections (“CCS”) than predicted values. Intramolecular interactions such as salt-bridges or charge solvation by backbone amide groups can cause conformation contractions leading to experimental collision cross sections which are smaller than predicted. N-terminal acetylation and a basic C terminal residue (His, Arg, Lys) may stabilize the secondary structure.
The study of Electron Transfer Dissociation (“ETD”) fragment ion structures using ion mobility mass spectrometry can be used to determine how charge-carrying amino acid residues affect the structure of solvent-free peptide cations and of radical cations. ETD may also be used to establish the effect of N-terminal acetylation on the structure of peptides containing amino acid side chains prone to form intramolecular interactions.
In order to perform detailed studies of ETD fragment ion structures it is necessary to make accurate measurements of the ion mobility drift times of ETD fragment ions and to correlate the measured ion mobility drift time with a collision cross section (“CCS”) of the ions.
It is known to use either a multi-point external calibration or a multi-point internal calibration method to determine a calibration function which relates the experimentally determined ion mobility drift time with a collision cross section (“CCS”).
The known techniques determine a calibration function which is then used during a subsequent ion mobility experiment.
However, during the course of an ion mobility experiment there may be a slight change in one or more instrument parameters (e.g. temperature, pressure etc.) and these can affect the measured ion mobility drift time during the ion mobility experiment.
As a result, current techniques do not enable very detailed ion mobility experiments to be performed which seek to understand very precise or small conformational changes in e.g. the structure of peptides having slightly different sequences.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.